In the art, various devices with respect to the condition of mold oscillation have been contemplated as technologies for increasing the lubrication in the mold in continuous casting. For example, in Japanese Patent Application Publication S61-20653 and Japanese Patent Application Publication S60-87955, there is disclosed a mold oscillation method utilizing a non-sine oscillation wave in which the upward speed of the mold is lower than the downward speed and, in Japanese Patent Application Publication H06-15425, there is disclosed an oscillation method in which the frequency or oscillation amplitude of the mold is controlled so that an appropriate negative strip time may be maintained for each steel grade. Further, Japanese Patent Application Publication H08-19845 discloses an oscillation technique involving a high-speed wave of oscillation equal to or higher than 40 mm/s during the upward period in mold oscillation, and Japanese Patent Application Publication H08-187562 discloses a method comprising increasing the oscillation amplitude according to the increase in casting speed while maintaining the frequency of mold oscillation within a certain range.
However, the inventions cited above are all directed only to devices concerning the condition of mold oscillation and therefore each naturally has its limits with respect to the effect of reducing the friction force between the mold and the billet.
Concerning this problem, the present inventors proposed, in Japanese Patent No. 3,298,586, a technology substituting for an improvement in lubrication by the above-mentioned oscillation alone. Thus, they proposed a continuous casting machine in which a mechanism having structural play, or an allowance of free motion, incorporated in the driving mechanism for pinch rolls is used to decrease the withdrawing speed during the upward movement of the mold and increase the withdrawing speed during the downward movement of the mold and thereby reduce the friction force between the mold and the billet in the mold. By using the continuous casting machine disclosed in the document cited above, it becomes possible to markedly reduce the friction force in question. For producing that effect stably, however, there is still room for improvement from the technological viewpoint.